Spinal fusion surgery usually involves placement of pedicle screws in lumbar and thoracic spine. Pedicle screws are typically available in different length and diameter and are used for a wide variety of indications such as open conventional surgery and minimally invasive spinal surgery. Typically, a longitudinal member is fastened to two or more vertebral anchors with the help of set screws or others similar fasteners on either side of the spine. Occasionally, the height of the screws inserted in to the vertebrae does not match up, particularly if more than two screws are inserted on each side. Uneven height of the head of the pedicle screw makes it harder for installation of the longitudinal member. If such a situation is encountered, the surgeon typically addresses it by utilization of reduction tools or a washer to raise the height of the pedicle screw. Occasionally, a previously inserted pedicle screw may need to be backed out a few turns such that the longitudinal member may be well seated inside the tulip of the pedicle screw. These manures can be tedious and time consuming. Loss of fixation is also a concern if a screw has to be backed out for alignment purposes. In minimally invasive cases, height alignment of the screws is even more critical because of limited access to the hardware.
It will be advantageous to have adjustable height pedicle screws, which allow elongation or shortening such that some of the aforementioned difficulties encountered during spinal instrumentation can be mitigated.
Most current designs of pedicle screws have a fixed height of the head of the screw in relation to the screw shaft. Intraoperative height adjustment is sometimes necessary for anchoring the longitudinal members, particularly in patients requiring multi-segmental fixation. Most surgeons tend to either utilize washers or back out the pedicle screws when there is a height mismatch. Backing out the screws may compromise the quality of fixation. Additionally, it is difficult to adjust the height of the pedicle screw during minimally invasive spinal surgery with currently available devices. Finally, it may be also be advantageous to further adjust the height of the pedicle screws after placement of the longitudinal member but before final tightening of the construct.